1. Field of the Invention
This invention pertains to the coating of air-permeable containers with a foil of thermoplastic material.
2. Description of the Prior Art
It is known in the prior art how to coat air-permeable containers, e.g., paper cups, with a foil of thermoplastic material using the following procedure: the container to be coated is placed in a deep-drawing mold; the foil is stretched in a hot plastic condition over the container opening; a differential pressure is then applied to the foil (low pressure from the side of the container or excess pressure from a source, through a cover, arranged over or above the foil and mold opening) to deep draw the foil into the container; and the foil is allowed to solidify in contact with the inner surface of the container. The above-described procedure works suitably well in applications where there are appreciable time intervals to permit suitable softening and then solidifying of the foil and where the applied differential pressure does not have to be too great while the foil is in its heated and soft condition.
Difficulties arise, however, using the above-described prior art procedure to process very thin foils, which would require processing within a very narrow range of temperatures, which would require very close limits on the amount of pressure differential to be applied, and which would require long periods of time for each container coating operation, thereby making it impractical to consider high machine speeds normally associated with mass production.
Further, even with very close and carefull control, processing such foils with a low pressure acting from only the underside of the container is often not sufficient to accomplish deep drawing. Therefore, instead of or in addition to creating a low-pressure vacuum condition, it becomes necessary to apply compressed air from the other side. To prevent this compressed air from quickly cooling the foil below the prescribed deep-drawing temperature, the air must be pre-heated or warmed. Accordingly, in this warm air environment, the foil does not solidify in its deep-drawn shape because it has no way of emitting heat to the surrounding air. The porous walls of the container do conduct some heat from the foil, but they do so slowly and inefficiently, since the only means of heat conduction from the container walls, other than to the heated air, is to the walls of the mold. It is evident that such slow cooling is incompatible with short-phase operation of high speed machines. Typically, the deep-drawing occurrences in the foil coating of 100 containers per minute, would each last for about one-half second, only part of which is available for the cooling of the foil in the deep-drawn condition. Should a mold be opened before the foil is completely solidified in the desired shape, it is possible that the inherent residual forces would re-shape the foil, particcularly in the concave corner ranges of the porous container. Thus, the finished product would not have foil completely fitting into these corners.
Therefore, it is a feature of this invention to provide an improved coating process for an air-permeable container, especially a paper cup, which presses a foil of thermoplastic material in the hot-plastic condition into the container through the use of a stream of gas under pressure, the pressure being greater than atmospheric on the open side of the container, which gas stream is then cooled until the foil is solidified in place in the container.
It is another feature of this invention to provide an improved process for deep drawing a foil into an air-permeable container and solidifying it in place so that it substantially completely fits the corners of the container into which it has been drawn as a coating.